The present invention generally relates to semiconductor element modules, and more particularly to a semiconductor element module which is connected to a main body in which a plurality of signal level systems coexist.
Recently, the power supply voltage standard and the signal level which were standardized in semiconductor elements are changing from the 5 V system to the 3.3 V system due to the processes being employed to make the semiconductor elements. There is a possibility that the 3.3 V system may even change to a system of a lower voltage such as 2.5 V and 1.8 V. In addition, even in the low-voltage system such as the 3.3 V system, there are cases where a plurality of interface levels (signal levels) may coexist, and it is necessary to take measures for such cases in a semiconductor element module when employing a system of switching a plurality of signal levels using the same output circuit.
Conventionally, a memory element module which is an example of the semiconductor element module employs a circuit system such as the transistor-transistor logic (TTL) and the emitter-coupled logic (ECL). In addition, while the trend is for the power supply voltage standard and the signal level of the semiconductor element to change to the lower voltage, there are the case where different power supply voltages coexist and the case where a plurality of interface levels coexist in the low voltage system.
In the case where the different power supply voltages coexist, the TTL and ECL may coexist, but a measure such as providing an erroneous insertion preventing mechanism in relation to a printed circuit board and a connector is taken.
On the other hand, in a signal system having a plurality of interface levels, a module of elements which conform to the interface levels is produced for each printed circuit board because there does not exist an element which conforms to a plurality of interface levels.
In the low-voltage system, in addition to the low-voltage-TTL (hereinafter simply referred to as LV-TTL) which uses the low voltage as it is with the threshold levels of the 5 V system, there is for example a proposal to employ the terminated LV-TTL (hereinafter simply referred to as T-LVTTL) which makes the signal transfer with a smaller amplitude and at a higher speed. The threshold level of the 5 V system regards a voltage of 2.0 V or greater as a high level and a voltage of 0.8 V or less as a low level.
According to the T-LVTTL, an intermediate point of LV-TTL push-pull transistors in the 3.3 V system, for example, is coupled to a power supply of 1.5 V via a resistor having a resistance of 50 .OMEGA.. The threshold levels are set to 1.5 V.+-.0.2 V, so that it is possible to realize a high-speed transfer with a small amplitude. In other words, the threshold levels are set so that a voltage of 1.7 V of greater is regarded as a high level and a voltage of 1.3 V or less is regarded as a low level.
Therefore, there are cases where a plurality of interface levels exist, such as the case where the LV-TTL and the T-LVTTL coexist. In such cases, if an element which can conform to the plurality of interface levels by switching to different signal level systems is mounted on a module, there is a problem in that it is necessary to modify a signal arrangement of the module or, to set a control signal for the switching every time the switching is made.